Liquid containment storage tanks are frequently used to store hydrocarbon liquids. When the stored liquid is volatile or presents a risk of pollution through evaporation, the storage tank is often equipped with a floating roof, which floats on top of the stored liquid and moves up and down with the liquid level. Floating roofs greatly reduce liquid evaporation, preventing loss of the stored liquid and reducing pollution due to hydrocarbon evaporation into the atmosphere.
Such floating roofs are often provided with support legs which are usually spaced about twenty feet apart and provide support to the roof when the roof is not floating on stored liquid, such as when the tank is emptied or taken out of service for maintenance. These roofs are usually floated by pontoons which are secured to the roof support structure. However, such pontoon-floated roofs leave a vapor space above the liquid surface in the tank. Thus, evaporation will occur in the tank until the vapor space is saturated, at which point equilibrium between the vapor space and the liquid is reached.
However, there will be losses of the liquid stored in the tank, as vapor leaks through seams in the roof or around seals. Engineering of floating roofs attempts to eliminate such leakage losses, but the existence of a relatively volatile vapor space immediately under the roof makes absolute elimination of such losses impossible.
Elimination of the vapor space is possible by using a full contact floating roof. Existing full contact roofs include aluminum and steel roofs. Aluminum full contact roofs are usually comprised of panels bolted to an aluminum framework. Such panels may comprise expanded aluminum honeycomb, or a foam core sandwiched between two layers of aluminum sheet. Most full contact steel roofs are constructed from steel plate welded together and surrounded by steel pontoons. Other, “pan type” steel roofs are simple flat plate welded together with a vertical rim along the edge.
However, these types of full contact roofs have engineering and practicality limitations. Current full contact roof designs are only marginally capable of sustaining the loads imposed on the structures. They are also easily upset and sunk if there is a large operations anomaly in the underlying tank. Because these roofs have to be constructed in the field, there are high labor and heavy-equipment machinery costs associated with assembling and moving materials around at the construction site. Further, steel roofs require periodic repainting and are very susceptible to corrosion, creating high maintenance costs and potentially limiting the useful life of the roof.
A further limitation of the aluminum honeycomb or foam core sandwiched-panel type roofs is the inability to test the individual honeycomb cells for the presence of a foreign or combustible vapor. Such vapor may be present if there is a leak in the outer sheeting cover. Moreover, the aluminum honeycomb or foam core sandwiched panels are normally joined to the outer aluminum sheeting cover with glue or adhesive that frequently becomes brittle and inflexible after being applied. Cyclic operation of the floating roof, or certain external loading conditions on the outer sheeting cover, such as walking on the roof, often cause theis glue or adhesive to crack, forming vapor or liquid paths between the individual compartments. Thus, the leak-tight integrity of the individual compartments may be compromised.
Accordingly, it is an object of the invention to provide a full contact floating roof which is full contact, yet is made of relatively lightweight, durable, and stable materials which are easy to assemble.
It is a further object of the invention to provide a full contact floating roof which is difficult to upset and sink.
It is another object of the invention to provide a full contact floating roof which provides additional options for fire protection over existing roofs.